AC-DC converter with output power suppression

ABSTRACT

A power conversion apparatus for converting AC power supplied from an AC power source to DC power includes an AC-DC conversion circuit connected with the AC power source at an input end thereof for converting the AC power to DC power, a DC-DC conversion circuit connected with an output end of the AC-DC conversion circuit at an input end thereof for converting DC voltage level of the DC power generated by the AC-DC conversion circuit, a smoothing capacitor parallel connected to the output end of the AC-DC conversion circuit and the input end of the DC-DC conversion circuit, a DC link voltage detector for detecting a voltage of the smoothing capacitor as a DC link voltage, and a control unit for controlling operation of the DC-DC conversion circuit. The control unit suppresses an output power of the DC-DC conversion circuit, if the DC link voltage is smaller than a predetermined value.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Division of application Ser. No.15/390,763, filed Dec. 27, 2016, which is a Division of application Ser.No. 14/667,894, filed Mar. 25, 2015 and claims priority to JapanesePatent Application No. 2014-68627, filed on Mar. 28, 2014, the entirecontents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power conversion apparatus forconverting AC power supplied from an AC power source to DC power.

2. Description of Related Art

Japanese Patent Application Laid-open No. 2006-129619 describes a powerconversion apparatus capable of preventing malfunctioning when theoutput current thereof is detected to have decreased under anenvironment where the output voltage of an AC power source variesgreatly. This power conversion apparatus is configured to set a commandvalue of the output current thereof below a predetermined value if theinput voltage supplied from the AC power source is detected to besmaller than a predetermined value, to prevent the output current frombeing determined to have decreased below the command value to therebyprevent malfunctioning.

However, since the power conversion apparatus is configured to determinethe command value of the output current based on the input voltage, atime necessary to make a determination whether the input voltage issmaller than the predetermined value may cause a problem. That is, sincethe input voltage is an AC voltage, and accordingly a peak value of theAC voltage has to be detected to determine the value of the inputvoltage, it may take a half-period of the AV voltage to make thedetermination at longest.

Furthermore, when the power conversion apparatus described in the abovepatent document is used for a system including an AC-DC converter and aDC-DC converter which are mounted on different units, communicationdevices for enabling communication between the AC-DC converter and theDC-DC converter are required. In this case, there may occur otherproblems such as degradation of responsiveness due to communicationdelay, increase in parts count due to mounting of communicationequipment, or reliability degradation due to interposition of thecommunication devices.

SUMMARY

An exemplary embodiment provides a power conversion apparatus forconverting AC power supplied from an AC power source to DC power,including:

an AC-DC conversion circuit connected with the AC power source at aninput end thereof for converting the AC power to DC power;

a DC-DC conversion circuit connected with an output end of the AC-DCconversion circuit at an input end thereof for converting DC voltagelevel of the DC power generated by the AC-DC conversion circuit;

a smoothing capacitor parallel connected to the output end of the AC-DCconversion circuit and the input end of the DC-DC conversion circuit;

a DC link voltage detector for detecting a voltage of the smoothingcapacitor as a DC link voltage; and

a control unit for controlling operation of the DC-DC conversioncircuit,

wherein the control unit is configured to suppress output power of theDC-DC conversion circuit if the DC link voltage is smaller than a firstpredetermined value.

According to the exemplary embodiment, there is provided a powerconversion apparatus capable of responding rapidly to voltage variationof an AC power source.

Other advantages and features of the invention will become apparent fromthe following description including the drawings and claim.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram of a power conversion apparatus according toa first embodiment of the invention;

FIG. 2 is a control block diagram showing output power suppressioncontrol performed by a control unit of the power conversion apparatusaccording to the first embodiment of the invention;

FIG. 3 is a flowchart showing steps of power conversion controlperformed by the control unit of the power conversion apparatusaccording to the first embodiment of the invention;

FIG. 4 is a graph showing an example of temporal variations of the DClink voltage, output power, input current and input voltage of the powerconversion apparatus according to the first embodiment of the inventionwhen the output power suppression control is performed;

FIG. 5 is a graph showing an example of temporal variations of the DClink voltage, output power, input current and input voltage of the powerconversion apparatus according to the first embodiment of the inventionwhen the output power suppression control is not performed;

FIGS. 6A and 6B are graphs showing other examples of temporal variationsof the DC link voltage, output power, input current and input voltage ofthe power conversion apparatus according to the first embodiment of theinvention when the output power suppression control is performed;

FIG. 7 is a control block diagram showing output power suppressioncontrol performed by a power conversion apparatus according to a secondembodiment of the invention;

FIG. 8 is a control block diagram showing output power suppressioncontrol performed by a power conversion apparatus according to a thirdembodiment of the invention;

FIG. 9 is a graph which a power conversion apparatus according to afourth embodiment of the invention uses to determine a coefficient k foroutput power suppression control based on the DC link voltage Vc;

FIG. 10 is a graph which a power conversion apparatus according to afifth embodiment of the invention uses to determine a coefficient k foroutput power suppression control based on the DC link voltage Vc;

FIG. 11 is a graph which a power conversion apparatus according to asixth embodiment of the invention uses to determine a coefficient k foroutput power suppression control based on the DC link voltage Vc; and

FIG. 12 is a circuit diagram of a power conversion apparatus accordingto a seventh embodiment of the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

In the below described embodiments, the same or equivalent sections orunits are indicated by the same reference numerals or characters.

First Embodiment

FIG. 1 is a circuit diagram of a power conversion apparatus according toa first embodiment of the invention. This power conversion apparatus isused for supplying electric power to a rechargeable battery byconverting AC power inputted from an AC power source such as a domesticpower source or a commercial AC power source to DC power.

As shown in FIG. 1, the power conversion apparatus includes an AC-DCconversion circuit 100, a smoothing capacitor 150 parallel-connected tothe AC-DC conversion circuit 100, a DC-DC conversion circuit 200, and acontrol unit 300. The AC-DC conversion circuit 100 is connected with anAC power source 400 at its input end, and connected with the input endof the DC-DC conversion circuit 200 at its output end. The output end ofthe DC-DC conversion circuit 200 is connected to a rechargeable battery500.

The AC-DC conversion circuit 100 includes a diode bridge circuit 10, afirst smoothing reactor 11 and a half-bridge circuit 12. The AC powersource 400 is connected to the diode bridge circuit 10 through the inputend of the AC-DC conversion circuit 100.

The diode bridge circuit 10 includes four diodes D1 to D4. The cathodesof the diode 1 and the diode 3 are connected to a first wire 15. Theanode of the diode D1 is connected to a first end of the AC power source400 and the cathode of the diode D2. The anode of the diode D3 isconnected to a second end of the AC power source 400 and the cathode ofthe diode D4. The anode of the diode D2 and the anode of the diode D4are connected to a second wire 16. The diode bridge circuit 10 and thehalf bridge circuit 12 are connected to each other by the first wire 15and the second wire 16. The first wire 15 is provided with the firstsmoothing reactor 11 between the diode bridge circuit 10 and the halfbridge circuit 12.

The half bridge circuit 12 includes a diode 5 and a switching element Q1constituted of a MOSFET. The diode 5 is connected to the high voltageside output end of the AC-DC conversion circuit 100 at its cathode, andconnected to the first wire 15 and the drain terminal of the switchingelement Q1 at its anode. The source terminal of the switching element Q1is connected to the second wire 16. The second wire 16 is connected tothe low voltage side output end of the AC-DC conversion circuit 100. Theswitching element Q1 includes a parasitic diode parallel-connectedreversely.

The DC-DC conversion circuit 200 includes a bridge circuit 20, atransformer 21 constituted of a first coil 20 and a second coil 21 b, adiode bridge circuit 22 and a second smoothing reactor 23.

The bridge circuit 20 includes switching elements Q2 and Q5 eachconstituted of a MOSFET. Each of the switching elements Q2 and Q4 isdisposed in the upper arm on the high voltage side. Each of theswitching elements Q3 and Q5 is disposed in the lower arm on the lowvoltage side. The switching element Q2 is connected to a high voltageside wire 24 at its drain terminal, and connected to the drain terminalof the switching element Q3 and one end of the first coil 21 a at itssource terminal. The switching element Q4 is connected to the highvoltage side wire 24 at its drain terminal, and connected to the drainterminal of the switching element Q5 and the other end of the first coil21 a at its source terminal. The source terminals of the switchingelements Q3 and Q5 are both connected to a low voltage side wire 25. Thehigh voltage side wire 24 and the low voltage side wire 25 are connectedto the high voltage side output end and the low voltage side output endof the AC-DC conversion circuit 100, respectively. Each of the switchingelements Q2 to Q5 includes a parasitic diode parallel-connectedreversely.

The diode bridge circuit 22 includes diodes D6 to D9. Each of the diodesD6 and D8 is disposed in the upper arm on the high voltage side. Each ofthe diodes D7 and D9 is disposed in the lower arm on the low voltageside. The diode D6 is connected to a high voltage side wire 26 at itscathode, and connected to the cathode of the diode D7 and one end of thesecond coil 21 b at its anode. The diode D8 is connected to the highvoltage side wire 26 at its cathode, and connected to the cathode of thediode D9 and the other end of the second coil 21 b at its anode. Theanodes of the diodes D7 and D9 are both connected to a low voltage sidewire 27. The high voltage side wire 26 is connected to the positiveelectrode of the rechargeable battery 500 through the second smoothingreactor 23. The low voltage side wire 27 is connected to the negativeelectrode of the rechargeable battery 500 through the second smoothingreactor 23.

The power conversion apparatus of this embodiment further includes afirst voltage detector 31, a current detector 32 and a second voltagedetector 33 as a DC link voltage detecting means.

The first voltage detector 31 is parallel-connected to the output end ofthe DC-DC conversion circuit 200 to detect the output voltage Vout ofthe DC-DC conversion circuit 200. The current detector 32 isparallel-connected to the output end of the DC-DC conversion circuit 200to detect the output current Iout of the DC-DC conversion circuit 200.The second voltage detector 33 is parallel-connected to the smoothingcapacitor 150 to detect the DC link voltage which is a voltage outputtedfrom the AC-DC conversion circuit 100 and applied to the smoothingcapacitor 150.

The control unit 300 includes a DC link voltage control section 34 and acoefficient calculation section 35. The control unit 300 receivesmeasurements of the output voltage Vout, the output current Iout and theDC link voltage Vc. The control unit 300 includes a memory in which apower command value Pout* is stored. The power command value Pout*commands the value of the output power Pout representing the poweroutputted from the output end of the DC-DC conversion circuit 200. Thecoefficient calculation section 35 calculates a coefficient k used foroutput power suppression control (control of the output power Pout), andsends it to the DC link voltage control section 34. The DC link voltagecontrol section 34 calculates a PWM signal based the output voltageVout, the output current Iout, the power command value Pout* and thecoefficient k. The control unit 300 monitors the DC link voltagerepresenting the value of the voltage applied to the DC-DC conversioncircuit 200. The control unit 300 stops controlling upon detecting thatthe DC link voltage Vc has fallen below a predetermined stop voltage.

FIG. 2 is a control block diagram showing the output power suppressioncontrol performed by the control unit 300 of the power conversionapparatus according to this embodiment.

The coefficient calculation section 35 calculates the coefficient kwhich is in the range between 0 and 1 based on the DC link voltage Vc,and sends the calculated coefficient k to the DC link voltage controlsection 34. The DC link voltage control section 34 obtains a compensatedpower command value Pout*′ by multiplying the power command value Pout*by the coefficient k. Subsequently, the DC link voltage control section34 obtains a compensated current command value Iout*′ by dividing thecompensated power command value Pout*′ by the output voltage Vout, andthen obtains an output current deviation dIout by subtracting thecompensated current command value Iout* from the output current Iout.The output current deviation dIout thus obtained is supplied to acurrent controller to generate the PWM signal to be applied to theswitching elements Q2 to Q5 as on/off signals.

Here, the coefficient k used for the output power suppression control isexplained in detail. The coefficient k is set to 1 when the DC linkvoltage Vc is larger than or equal a first predetermined value V1. Asthe DC link voltage Vc decreases from the first predetermined value V1to a second predetermined value V2, the coefficient k is decreasedmonotonically and linearly from 1 to 0. When the DC link voltage Vc issmaller than or equal to the second predetermined value V2, thecoefficient k is set to 0. That is, when the DC link voltage Vc issmaller than or equal to the second predetermined value V2, thecompensated power command value Pout*′ is 0 so that the output powerPout of the DC-DC conversion circuit 200 takes the minimum value withinits control range. The first predetermined value V1 is set to a valuesmaller than the value of the DC link voltage Vc when the AC powersource 400 is in the normal state minus the peak value of the ripplevoltage of the DC link voltage Vc. On the other hand, the secondpredetermined value V2 is set to a value larger than the predeterminedstop voltage below which the control unit 300 stops the control of theDC-DC conversion circuit 200.

FIG. 3 is a flowchart showing steps of power conversion controlperformed at regular time intervals by the control unit 300 of the powerconversion apparatus according to this embodiment. This control beginsin step S301 where the power command value Pout* is received. Asdescribed in the foregoing, the power command value Pout* may be readfrom the memory within the control unit 300. In subsequent step S302,the detection value of the DC link voltage Vc is received, and then itis determined whether or not the DC link voltage Vc is larger than orequal to the first predetermined value V1 in step S303. If thedetermination result in step S303 is affirmative, since the coefficientk is 1, the compensated power command value Pout*′ is set equal to thepower command value Pout in step S304. If the determination result instep S303 is negative, since the coefficient k is set to a value smallerthan 1 depending on the DC link voltage Vc, the compensated powercommand value Pout*′ is set equal to the power command value Pout*multiplied by the coefficient k in step S305. As a result, the controlof the output power Pout is performed in accordance with the compensatedpower command value Pout*′ in step S306.

FIG. 4 shows an example of temporal variations of the input voltage Vacsupplied from the AC power source 400, the input current Iac suppliedfrom the AC power source 400, the output power Pout and the DC linkvoltage Vc in a case where the output power suppression control isperformed when the voltage of the AC voltage 400 has dropped by 40%.FIG. 5 shows an example of temporal variations of the input voltage Vac,the input current Iac, the output power Pout and the DC link voltage Vcin a case where the output power suppression control is not performedwhen the voltage of the AC voltage 400 has dropped by 40%.

The DC link voltage Vc decreases with the decrease of the input voltageVac. In the example of FIG. 4, since the output power suppressioncontrol is performed, the output power Pout is suppressed. Because ofperforming the control to suppress the output power Pout, the decreaseof the DC link voltage Vc is also suppressed, and as a result, the DClink voltage Vc is prevented from falling below a lower limit valueVc_under and exceeding an upper limit value Vc_over. Here, the lowerlimit value Vc_under is a value which is smaller than the secondpredetermined value V2, and puts an inrush current caused due to thedifference between the output voltage of the AC-DC conversion circuit100 and the DC link voltage Vc within an allowable range when the inputcurrent Vac returns to its initial value. The upper limit value Vc_overis the withstand voltage of the smoothing capacitor 150.

In the example of FIG. 5, since the output power suppression control isnot performed, the output power Pout is constant, and the DC linkvoltage Vc falls below the lower limit value Vc_under. Accordingly, whenthe input voltage Vac returns to its initial value, an inrush currentIac occurs due to the difference between the output voltage of the AC-DCconversion circuit 100 and the DC link voltage Vc.

FIG. 6A shows an example of temporal variations of the input voltage Vacsupplied from the AC power source 400, the input current Iac suppliedfrom the AC power source 400, the output power Pout and the DC linkvoltage Vc in a case where the output power suppression control isperformed when a power failure has occurred. FIG. 6B shows an example oftemporal variations of the input voltage Vac supplied from the AC powersource 400, the input current Iac supplied from the AC power source 400,the output power Pout and the DC link voltage Vc in a case where theoutput power suppression control is performed when a voltage variationhas occurred.

In each of the examples of FIGS. 6A and 6B, the DC link voltage Vcdecreases with the decrease of the input voltage Vac. By performing theoutput suppression control, the output power Pout is suppressed link inthe example of FIG. 4. As a result, the decrease of the DC link voltageVc is suppressed, and the DC link voltage Vc is prevented from fallingbelow the lower limit value Vc_under.

The power conversion apparatus of this embodiment provides the followingadvantages.

When DC link voltage Vc is smaller than the first predetermined valueV1, since the output power Pout of the DC-DC conversion circuit 200 issuppressed in accordance with the value of the DC link voltage Vc, theDC link voltage Vc can be suppressed from decreasing excessively. Atthis time, since measurement of the voltage variation is performed forthe DC link voltage Vc which is a DC voltage, it is possible to detectthe voltage variation of the AC voltage source 400 rapidly compared towhen measurement of the voltage variation is performed directly for theoutput voltage of the AC voltage source 400. It is also possible toperform the control without measuring or using a voltage value or acurrent value of any point which is more to the side of the AC powersource 400 than the smoothing capacitor 150 is. Accordingly, in a casewhere the AC-DC conversion circuit 100 and the DC-DC conversion circuit200 are mounted on different apparatuses, it is possible to complete thecontrol of the DC link voltage Vc on the side of the apparatus on whichthe DC-DC conversion circuit 200 is mounted. Therefore, the speed orresponsiveness of the control to suppress the decrease of the DC linkvoltage Vc can be increased to thereby increase the robustness of thepower conversion apparatus.

If the first predetermined voltage V1 is determined without respect tothe ripple voltage, there may occur a case where the DC link voltage Vcfalls below the first predetermined value V1 due to the ripple voltageeven when the AC power source is in the normal state. In this case,since the control to suppress the output power of the DC-DC conversioncircuit 200 is performed although the AC power source 400 is operatingnormally, the efficiency of power supply by the DC-DC conversion circuit200 is lowered. According to the configuration described above, when theAC power source 400 is operating normally, the control to suppress theoutput power of the DC-DC conversion circuit 200 is not performedregardless of the ripple voltage, and so the efficiency of power supplyby the DC-DC conversion circuit 200 can be prevented from being lowered.

If the second predetermined value V2 is smaller than the predeterminedstop voltage, it may occur that the DC link voltage Vc falls below thestop voltage even when the control to suppress the output power Pout isperformed, causing the power conversion apparatus to stop operation byits low-voltage protection function. By setting the second predeterminedvalue V2 larger than the predetermined stop voltage, the DC link voltageVc can be prevented from falling below the predetermined stop voltagedue to performing the control to suppress the output power Pout, tothereby prevent the power conversion apparatus from stopping operationdue to performing the control to suppress the output power Pout.

Since the coefficient k is changed continuously, the compensated powercommand value Pout*′ can be changed continuously. As a result, when theDC link voltage Vc increases, the compensated power command value Pout*′increases, while when the DC link voltage Vc decreases, the compensatedpower command value Pout*′ decreases. This makes it possible to causethe DC link voltage Vc to converge to a value corresponding to thecompensated power command value Pout*′ and prevent the DC link voltageVc from oscillating.

Second Embodiment

Next, a second embodiment of the invention is described. The powerconversion apparatus according to the second embodiment differs frompower conversion apparatus according to the first embodiment in thecontrol performed by the DC link voltage control section 34. In thesecond embodiment, instead of the power command value Pout*, a voltagecommand value Vout* as a value to command the output voltage Vout of theDC-DC conversion circuit 200 is written in or read from the memoryprovided in the control unit 300.

FIG. 7 is a control block diagram showing the output power suppressioncontrol performed by the control unit 300 of the power conversionapparatus according to the second embodiment.

The coefficient calculation section 35 calculates the coefficient kwhich is in the range between 0 and 1 based on the DC link voltage Vc,and sends the calculated coefficient k to the DC link voltage controlsection 34. The DC link voltage control section 34 obtains a compensatedvoltage command value Vout*′ by multiplying the voltage command valueVout* by the coefficient k. Subsequently, the DC link voltage controlsection 34 obtains a voltage deviation dVout by subtracting thecompensated voltage command value Vout*′ from the output voltage Vout.Thereafter, a compensated current command value Iout*′ is obtained byinputting the obtained voltage deviation dVout to a PI controller, andthen an output current deviation dIout is obtained by subtracting thecompensated current command value Iout*′ from the output current Iout.The output current deviation dIout thus obtained is supplied to acurrent controller to generate the PWM signal to be applied to theswitching elements Q2 to Q5 as on/off signals.

The power conversion apparatus according to the second embodimentprovides advantages similar to those provided by the power conversionapparatus according to the first embodiment.

Third Embodiment

Next, a third embodiment of the invention is described. The powerconversion apparatus according to the third embodiment differs frompower conversion apparatus according to the first embodiment in thecontrol performed by the DC link voltage control section 34. In thethird embodiment, instead of the power command value Pout*, a currentcommand value Iout* as a value to command the output current Iout of theDC-DC conversion circuit 200 is written in or read from the memoryprovided in the control unit 300.

FIG. 8 is a control block diagram showing the output power suppressioncontrol performed by the control unit 300 of the power conversionapparatus according to the third embodiment.

The coefficient calculation section 35 calculates the coefficient kwhich is in the range between 0 and 1, and sends the calculatedcoefficient k to the DC link voltage control section 34. The DC linkvoltage control section 34 obtains the compensated current command valueIout*′ by multiplying the current command value Iout* by the coefficientk. Subsequently, the DC link voltage control section 34 obtains thecurrent deviation dIout by subtracting the compensated current commandvalue Iout*′ from the output current Iout. The output current deviationdIout thus obtained is supplied to a current controller to generate thePWM signal to be applied to the switching elements Q2 to Q5 as on/offsignals.

In this embodiment, since the control unit 300 can operate without usingthe output voltage Vout, the first voltage detector 31 may be omitted.

The power conversion apparatus according to the third embodimentprovides advantages similar to those provided by the power conversionapparatus according to the first embodiment.

Fourth Embodiment

Next, a fourth embodiment of the invention is described. The powerconversion apparatus according to the fourth embodiment differs from thepower conversion apparatuses according to the above embodiments in theprocess for obtaining the coefficient k by the coefficient calculationsection 35.

FIG. 9 is a graph showing a relationship between the DC link voltage Vcand the coefficient k, which the coefficient calculation section 35 ofthis embodiment uses to calculate the coefficient k.

When the DC link voltage Vc is larger than the first predetermined valueV1, the coefficient k is set to 1. As the DC link voltage Vc decreasesfrom the first predetermined value V1 to the second predetermined valueV2, the coefficient k is decreased from 1 to 0 exponentially, orquadratic or larger order-functionally. That is, the decrease rate ofthe coefficient k increases with the increase of the DC link voltage Vc.The coefficient k is set to 0 when the DC link voltage Vc is smallerthan the second predetermined value V2.

The power conversion apparatus according to the fourth embodimentprovides advantages similar to those provided by the power conversionapparatus according to the first embodiment.

Fourth Embodiment

Next, a fifth embodiment of the invention is described. The powerconversion apparatus according to the fifth embodiment differs from thepower conversion apparatuses according to the above embodiments in theprocess for obtaining the coefficient k by the coefficient calculationsection 35.

FIG. 10 is a graph showing a relationship between the DC link voltage Vcand the coefficient k, which the coefficient calculation section 35 ofthis embodiment uses to calculate the coefficient k. In this embodiment,the coefficient k is changed stepwise with the increase of the DC linkvoltage Vc.

When the DC link voltage Vc is larger than the first predetermined valueV1, the coefficient k is set to 1. When the DC link voltage Vc issmaller than or equal to the first predetermined value V1, and largerthan a value Vα which is smaller than the first predetermined value V1and larger than the second predetermined value V2, the coefficient k isset to k1 which is smaller than 1 and larger than 0. When the DC linkvoltage Vc is smaller than or equal to Vα and larger than a value Vβwhich is smaller than Vα and larger than the second predetermined valueV2, the coefficient k is set to k2 which is smaller than k1 and largerthan 0. When the DC link voltage Vc is smaller than or equal to Vβ andlarger than a value Vγ which is smaller than Vβ and larger than thesecond predetermined value V2, the coefficient k is set to k3 which issmaller than k2 and larger than 0. When the DC link voltage Vc issmaller than or equal to Vγ and larger than the second predeterminedvalue V2, the coefficient k is set to k4 which is smaller than k3 andlarger than 0. When the DC link voltage Vc is smaller than or equal tothe second predetermined value V2, the coefficient k is set to 0.

Here, the coefficient k can be set in arbitrary number of steps betweenthe first predetermined value V1 and the second predetermined value V2.These steps may be the same as one another or different from one anotherin step width of the coefficient k and step width of the DC link voltageVc.

The power conversion apparatus according to the fifth embodimentprovides the following advantages in addition to the same advantages asthose provided by the power conversion apparatus according to the firstembodiment.

To determine the coefficient k1 using a predetermined function, theprocessing load of the control unit 300 increases, because an arithmeticprocessing or a mapping process has to be carried out. In the powerconversion apparatus according to the fifth embodiment, since such anarithmetic processing can be reduced and such a mapping process can beeliminated, the processing load of the control unit 300 and the capacityof the memory provided in the control unit 300 can be reduced.

Sixth Embodiment

Next, a sixth embodiment of the invention is described. The powerconversion apparatus according to the sixth embodiment differs from thepower conversion apparatuses according to the above embodiments in theprocess for obtaining the coefficient k by the coefficient calculationsection 35.

FIG. 11 is a graph showing a relationship between the DC link voltage Vcand the coefficient k, which the coefficient calculation section 35 ofthis embodiment uses to calculate the coefficient k. In this embodiment,the coefficient k is changed stepwise with the increase of the DC linkvoltage Vc like in the fifth embodiment, and hysteresis is providedbetween when the coefficient k is reduced stepwise and when thecoefficient k is increased stepwise.

When the DC link voltage Vc is larger than the first predetermined valueV1, the coefficient k is set to 1. When the DC link voltage Vc decreasesfrom the first predetermined value V1 to a value Vα′ which is smallerthan the first predetermined value V1 and larger than the secondpredetermined value V2, the coefficient k is set to 1. When the DC linkvoltage Vc decreases from Vα′ to a value Vβ′ which is smaller than Vα′and larger than the second predetermined value V2, the coefficient k isset to k1 which is smaller than k1 and larger than 0. When the DC linkvoltage Vc decreases from Vβ′ to a value Vγ′ which is smaller than Vβ′and larger than the second predetermined value V2, the coefficient k isset to k2 which is smaller than k1 and larger than 0. When the DC linkvoltage Vc decreases from Vγ′ to a value VΔ′ which is smaller than Vγ′and larger than the second predetermined value V2, the coefficient k isset to k3 which is smaller than k2 and larger than 0. When the DC linkvoltage Vc decreases from VΔ′ to the second predetermined value V2, thecoefficient k is set to k4 which is smaller than k3 and larger than 0.When the DC link voltage Vc falls below the second predetermined valueV2, the coefficient k is set to 0.

On the other hand, when the DC link voltage Vc increases from the secondpredetermined value V2 to VΔ′, the coefficient k is set to 0. When theDC link voltage Vc increases from Vα′ to Vγ′, the coefficient k is setto k4. When the DC link voltage Vc increases from Vγ′ to Vβ′, thecoefficient k is set to k3. When the DC link voltage Vc increases fromVβ′ to Vα′, the coefficient k is set to k2. When the DC link voltage Vcincreases from Vα′ to the first predetermined value V1, the coefficientk is set to k1. When the DC link voltage Vc exceeds the firstpredetermined value V1, the coefficient k is set to 1.

Here, the coefficient k can be set in arbitrary number of steps betweenthe first predetermined value V1 and the second predetermined value V2as in the fifth embodiment. These steps may be the same as one anotheror different from one another in step width of the coefficient k andstep width of the DC link voltage Vc.

The power conversion apparatus according to the sixth embodimentprovides the following advantages in addition to the same advantages asthose provided by the power conversion apparatus according to the fifthembodiment.

When the coefficient k is increased or decreased, there is a concernthat hunting of the coefficient k may occur in response to a slightchange of the DC link voltage Vc, causing the output power Pout to vary.Since hysteresis is provided between when the coefficient k is reducedstepwise and when the coefficient k is increased stepwise, hunting ofthe coefficient k can be prevented to thereby prevent the output powerPout from varying.

Seventh Embodiment

Next, a seventh embodiment of the invention is described. FIG. 12 is acircuit diagram of a power conversion apparatus according to the seventhembodiment of the invention. The power conversion apparatus according tothe seventh embodiment differs from the power conversion apparatusaccording to first embodiment in the structure of the AC-DC conversioncircuit 100.

In this embodiment, the AC-DC conversion circuit 100 includes asmoothing reactor 13 constituted of a first reactor 13 a and a secondreactor 13 b, and a bridge circuit 14. The AC source 400 is connected tothe first reactor 13 a and the second reactor 13 b through the input endof the AC-DC conversion circuit 100.

The bridge circuit 14 includes a diode D5 a, a diode D5 b, and switchingelements Q1 a and Q1 b each constituted of a MOSFET Each of the diodesD5 a and D5 b is disposed in the upper arm on the high voltage side.Each of the switching elements Q1 a and Q1 b is disposed in the lowerarm on the low voltage side. The diode D5 a is connected to a highvoltage side wire 15A at its cathode, and connected to the drainterminal of the switching element Q1 a and the first reactor 13 a at itsanode. The diode D5 b is connected to the high voltage side wire 15A atits cathode, and connected to the drain terminal of the switchingelement Q1 b and the second reactor 13 b at tis anode. The sourceterminals of the switching elements Q1 a and Q1 b are both connected toa low voltage side wire 16A. The high voltage side wire 15A and the lowvoltage side wire 16A are connected to the high voltage side output endand the low voltage side output end of the AC-DC conversion circuit 100,respectively.

In this embodiment, the control unit 300 performs control in the waysimilar to those performed in the above embodiments.

The power conversion apparatus according to the seventh embodimentprovides advantages similar to those provided by the power conversionapparatus according to the first embodiment.

Modifications

In each of the above embodiments, the output end of the DC-DC conversioncircuit 200 is connected with the rechargeable battery 500. However, theoutput end of the DC-DC conversion circuit 200 may be connected with acurrent consuming load other than a rechargeable battery. The structureof the AC-DC conversion circuit 100 is not limited to the one describedin any of the above described embodiments. The AC-DC conversion circuit100 may have any structure capable of converting received AC power to DCpower to be supplied to the DC-DC conversion circuit 200.

The structure of the DC-DC conversion circuit 200 is not limited to theone described in any of the above described embodiments. The DC-DCconversion circuit 200 may have any structure capable of changing thelevel of the DC voltage supplied from the AC-DC conversion circuit 100.

The circuit elements used in the first and seventh embodiment are diodesor MOSFETs. However, IGBTs or bipolar transistors may be used as theircircuit elements.

In the first and seventh embodiments, the AC power source 400 is asingle phase power source. However, the AC power source 400 may be athree or more phase power source.

The above explained preferred embodiments are exemplary of the inventionof the present application which is described solely by the claimappended below. It should be understood that modifications of thepreferred embodiments may be made as would occur to one of skill in theart.

What is claimed is:
 1. A power conversion apparatus comprising: an AC-DCconversion circuit connected with an AC power source at an input endthereof for converting an AC power supplied from the AC power source toDC power; a DC-DC conversion circuit connected with an output end of theAC-DC conversion circuit at an input end thereof for converting DCvoltage level of the DC power generated by the AC-DC conversion circuit;a smoothing capacitor parallel connected to the output end of the AC-DCconversion circuit and the input end of the DC-DC conversion circuit; aDC link voltage detector for detecting a voltage of the smoothingcapacitor as a DC link voltage; and a control unit for controllingoperation of the DC-DC conversion circuit, wherein the control unit isconfigured to suppress output power of the DC-DC conversion circuit ifthe DC link voltage is smaller than a first predetermined value, and thecontrol unit suppress the output power of the DC-DC conversion circuitby multiplying a current command value as a value to command an outputcurrent of the DC-DC conversion circuit by a coefficient set in a rangebetween 0 and 1 in accordance with the DC link voltage.